Anyway to use light information as a mask?
Sort of. 

Create a light. Create an Empty and use the Copy Location constraint with the light as the target.

Use the following node tree as the texture for the object. Be sure to note the Empty selected as the Object in the Texture Coordinate node.



In this case, I'm just mixing between red and blue. Transparency doesn't seem to work, but you could create a second view layer, use this texture with an emission shader and use that as a mask in the compositor.